1. Field of the Invention
The present invention relates to an autolensmeter, more particularly to an autolensmeter for automatically measuring refracting power of lens such as those for eyeglasses or contact lens, i.e., refracting powers such as spherical power, cylindrical power, inclination of cylindrical axis and prism power.
2. Description of the Prior Art
When a person has normal eyesight, the images of a distant object or the like is accurately formed on the retina of his eyes. However, not all human eyes are normal and a variety of eyeglasses or contact lens are required depending on whether a person is short-sighted, far-sighted or the like. On the other hand, if a spectacle maker or eyeglass factory makes eyeglasses, it is required that lens having a specific refracting power are provided according to the visual acuity of a specific person. For that purpose, an autolensmeter has been developed and has practically been used to determine refracting power of optical systems such as a lens for eyeglasses or the like.
An example of such autolensmeter is disclosed in Japanese Patent laid-open No. 57-29923 filed by Hiroshi Tamaki and assigned to same assignee of this application. The apparatus is a so-called non-image formation type one in which a means for detecting light is disposed before the focal point of a lens to be examined. According to this apparatus, the refracting power may be determined by observing the shape and/or position of a projected pattern received by the means for detecting light and comparing them with those of the mask pattern per se to estimate change or deformation observed therebetween.
In such apparatus, the width of the mask pattern should be narrowed so as to assure sharpness of the pattern projected on a photodetecting surface of the photodetecting means. This, in turn, leads to the reduction in the width of an effective luminous flux used to determine refracting power of lens to be examined. Therefore, it is needed to use expensive elements such as a highly sensitive element for detecting light or a light source of high luminance because of the low quantity of light projected on the photodetecting means.
Moreover, in the apparatus of this type, there are various factors which influences on the accuracy of measurement, such as external scattering factors inclusive of deposition of dusts and/or water drops on the lens and/or the mask pattern.